板材自阻加热成形中电流引发温度场及载荷和变形的极性效应与机理

Aiming at the phenomenon of the polarity effect which would happen in the process of metal resistance heating forming, a research project of summarizing the rules and revealing the mechanism of the polarity effect is presented. The polarity effect that relates to the temperature field, load and deformation in the resistance heating forming process of several kinds of light alloys would be studied, and the polarity effect producing in the motion direction of the free electrons would be confirmed and characterized, and especially the abnormal phenomenon of the polarity effect should be observed and revealed. Study the energy level gaps of the free electrons and the energy change of the electron transition of the electrode materials and heating sheets, and try to establish their relationship with cathode-anode temperature difference, and find out the distinction of the cathode-anode temperature difference effect especially when in the two extreme cases that the difference in atomic number of the electrode materials and the deformation blank is small or large. Study the polarity effect when the light metal and light alloys with different grain size and grain shape are during resistance heating tensile and bulging, and reveal the laws of the effect of the electric current on the resistance heating tensile and bulging of the metal materials which deform by the two primary deformation mechanism including dislocation glide and grain boundary sliding. And the in-situ electron micro-technique would be used in the light alloy specimen’ electric current auxiliary tensile test. The micro-mechanism of the polarity effect would be revealed by in-situ TEM observation and confirming the motion of the dislocation during the application of electric field, besides, considering the driving force for the atomic diffusion given by the electric current, which would provide the basis for resistance heating forming process design.

针对金属板材自阻加热成形时存在的极性效应，提出总结极性效应规律、揭示其机理的研究课题。研究不同种类轻金属及其合金板材自阻加热成形中温度场、载荷、变形的极性效应，确定并表征在自由电子运动方向上产生的极性效应，捕捉极性反常现象并揭示原因；探明电极和加热板材的自由电子能级差和电子跃迁能量变化与正负极温度差的关系，尤其是电极材料原子序数与变形坯料材料相近或相差较大的极端情况下正负极温度差效应的区别；研究不同晶粒尺度和形态对于轻金属及其合金板材自阻加热条件下拉伸和胀形的极性效应，确定电流对于材料在分别以位错滑移或者晶界滑动为主变形机制的条件下自阻加热拉伸和胀形的影响规律；将用原位电子显微技术进行试件施加电流情况下拉伸实验。通过透射电镜原位观察和确认轻金属中的位错在通电的情况下运动情况，并考察电流对于原子扩散的驱动，从而揭示极性效应的微观机理并为自阻加热成形工艺设计提供依据。

通过对多种金属和合金板材的电辅助加热、电辅助拉伸、电辅助胀形、微观观察、电迁移理论分析的研究，取得了如下具有一定科学意义的结果。.自阻加热的板料靠近负极区域的温度高于正极7K左右，该现象与帕尔贴效应有关，铜电极中自由电子所占能级高于镁合金，因此在负极一侧，大量的电子通过接触面由紫铜电极进入到镁合金中，此过程电子由高能级向低能级跃迁，释放能量；在正极一侧则相反，最终导致温差的出现。在电辅助拉伸过程中，当电流方向与拉伸方向相同时，漂移电子撞击位错产生与拉伸方向相反的电子风力，该力作为阻碍力附加在拉伸力上，而当电流方向与拉伸方向相反时，电子风力方向与拉伸变形方向相同，其作为推动力附加在拉伸力上，因此电流方向发生改变时，材料变形应力也发生改变，影响达到20%左右。自阻加热自由胀形得到的半球顶偏向正极一侧，偏移率在直径的5%-10%。这是由于在材料的变形过程中，偏向负极端的材料所受电子风力方向与材料的流动方向相同，电子风力作为一种推动力促进材料的变形。由于位错散射电子产生的剩余电阻在电流作用下会通过焦耳热效应生成额外热量使得位错能量升高，从而减少位错运动的激活能；漂移电子撞击位错会产生电子风力，电子风力可以降低位错运动派-纳力，打开位错缠结，使位错沿电流方向平行排列，从而提高位错的可动性，并会通过提高原子的扩散能力来加速位错的攀移，从而改善材料的塑性。以AZ31镁合金为例，其轧制态的位错密度高达1015m-2，因此材料在自阻加热变形过程中会受到沿板材切向由负极指向正极的0.37N/mm3的额外体积力。由于电迁移效应对溶质原子扩散的贡献和漂移电子与空位交互作用对扩散溶质原子扩散能力的增强，δ′强化相的析出和粗化过程将得到促进，从而缩短材料达到峰时效状态的时间，并降低达到峰时效的处理温度。根据扩散动力学和电迁移理论，给出了通电状态下材料内部δ′相颗粒的的粗化动力学方程。.发表学术论文14篇，获得授权发明专利4项，开发的电辅助成形技术与系统在航天领域应用，典型零件已经批产，装机运行。